Pope says Trump 'not Christian,' Trump calls comment 'disgraceful'

Pope Francis assailed Republican presidential candidate Donald Trump's views on U.S. immigration as "not Christian" on Thursday, prompting the billionaire businessman to reprimand the religious leader as "disgraceful" for questioning his faith.

No stranger to controversy, Trump, the longtime party front-runner in national opinion polls, has vowed if elected president to build a wall between the United States and Mexico to keep out immigrants who enter illegally.

In a freewheeling conversation on his flight home from a visit to Mexico, Francis told reporters, "A person who thinks only about building walls, wherever they may be, and not building bridges, is not Christian."

Click through images of Pope Francis' visit to Mexico:

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Pope Francis visits Mexico

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Pope says Trump 'not Christian,' Trump calls comment 'disgraceful'

Bishops attend the open-air mass by Pope Francis in Ecatepec --a rough, crime-plagued Mexico City suburb-- on February 14, 2016. Pope Francis has chosen to visit some of Mexico's most troubled regions during his five-day trip to the world's second most populous Catholic country. AFP PHOTO/GABRIEL BOUYS / AFP / GABRIEL BOUYS (Photo credit should read GABRIEL BOUYS/AFP/Getty Images)

ECATEPEC, MEXICO - FEBRUARY 14: People sings during a mass at Ecatepec on February 14, 2016 in Ecatepec, Mexico. Pope Francis is on a five-day visit in Mexico from February 12 to 17 where he is expected to visit five states. (Photo by Hector Vivas/LatinContent/Getty Images)

A young girl waits for the passage of Pope Francis on his way to the Nuncianture, in Mexico City on February 13, 2016. Francis became the first pope to enter Mexico's National Palace to meet President Enrique Pena Nieto, as he starts a cross-country tour that will highlight the country's violence and migration troubles. AFP PHOTO. AFP PHOTO/MARIO VAZQUEZ / AFP / MARIO VAZQUEZ (Photo credit should read MARIO VAZQUEZ/AFP/Getty Images)

Catholic faithful wait for the arrival of Pope Francis in Ecatepec --a rough, crime-plagued Mexico City suburb-- where he is to celebrate an open-air mass, on February 14, 2016. Pope Francis has chosen to visit some of Mexico's most troubled regions during his five-day trip to the world's second most populous Catholic country. AFP PHOTO/ JULIO CESAR AGUILAR / AFP / Julio Cesar Aguilar (Photo credit should read JULIO CESAR AGUILAR/AFP/Getty Images)

Pope Francis officiates an open-air mass in Ecatepec, near Mexico City, on February 14, 2016. Pope Francis urged Mexicans on Sunday to turn their country into a land of opportunity where there is no need to emigrate or mourn victims 'of the merchants of death.' AFP PHOTO/ALFREDO ESTRELLA / AFP / ALFREDO ESTRELLA (Photo credit should read ALFREDO ESTRELLA/AFP/Getty Images)

ECATEPEC, MEXICO - FEBRUARY 14: Pope Francis wlaks with a censer during a mass for the people at Ecatepec on February 14, 2016 in Ecatepec, Mexico. Pope Francis is on a five days visit in Mexico from February 12 to 17 where he is expected to visit five states. (Photo by Hector Vivas/LatinContent/Getty Images)

Pope Francis waves from the popemobile on his way to the cathedral, in Mexico City on February 13, 2016. Francis became the first pope to enter Mexico's National Palace to meet President Enrique Pena Nieto, as he starts a cross-country tour that will highlight the country's violence and migration troubles. AFP PHOTO/GABRIEL BOUYS / AFP / GABRIEL BOUYS (Photo credit should read GABRIEL BOUYS/AFP/Getty Images)

Catholic faithfuls take communion during an open-air mass by Pope Francis in Ecatepec --a rough, crime-plagued Mexico City suburb-- on February 14, 2016. Pope Francis has chosen to visit some of Mexico's most troubled regions during his five-day trip to the world's second most populous Catholic country. AFP PHOTO / ALFREDO ESTRELLA / AFP / ALFREDO ESTRELLA (Photo credit should read ALFREDO ESTRELLA/AFP/Getty Images)

Pope Francis waves at the crowd from the Popemobile after celebrating an open-air mass in Ecatepec, near Mexico City on February 14, 2016. Pope Francis urged Mexicans on Sunday to turn their country into a land of opportunity where there is no need to emigrate or mourn victims 'of the merchants of death.'. AFP PHOTO/ JULIO CESAR AGUILAR / AFP / Julio Cesar Aguilar Fuentes (Photo credit should read JULIO CESAR AGUILAR FUENTES/AFP/Getty Images)

Pope Francis waves from the popemobile upon arrival in Ecatepec --a rough, crime-plagued Mexico City suburb-- where he is to celebrate an open-air mass, on February 14, 2016. Pope Francis has chosen to visit some of Mexico's most troubled regions during his five-day trip to the world's second most populous Catholic country. AFP PHOTO / YURI CORTEZ / AFP / YURI CORTEZ (Photo credit should read YURI CORTEZ/AFP/Getty Images)

ECATEPEC, MEXICO - FEBRUARY 14: Faithful prays after receiving the communion wafer during a mass for the people at Ecatepec on February 14, 2016 in Ecatepec, Mexico. Pope Francis is on a five days visit in Mexico from February 12 to 17 where he is expected to visit five states. (Photo by Hector Vivas/LatinContent/Getty Images)

A security member stands guard next to a graffiti depicting Pope Francis in Ecatepec --a rough, crime-plagued Mexico City suburb-- where he will celebrate an open-air mass, on February 14, 2016. Pope Francis has chosen to visit some of Mexico's most troubled regions during his five-day trip to the world's second most populous Catholic country. AFP PHOTO / YURI CORTEZ / AFP / YURI CORTEZ (Photo credit should read YURI CORTEZ/AFP/Getty Images)

Faithfuyl welcome Pope Francis as he arrives to celebrate an open-air Holy Mass in the Basilica of Our Lady of Guadalupe in Mexico on February 13, 2016. Pope Francis is in Mexico for a trip encompassing two of the defining themes of his papacy: bridge-building diplomacy and his concern for migrants seeking a better life. AFP PHOTO / GABRIEL BOUYS / AFP / GABRIEL BOUYS (Photo credit should read GABRIEL BOUYS/AFP/Getty Images)

Pope Francis arrives to celebrate a Holy Mass in the Basilica of Our Lady of Guadalupe in Mexico on February 13, 2016. Pope Francis is in Mexico for a trip encompassing two of the defining themes of his papacy: bridge-building diplomacy and his concern for migrants seeking a better life. AFP PHOTO / GABRIEL BOUYS / AFP / GABRIEL BOUYS (Photo credit should read GABRIEL BOUYS/AFP/Getty Images)

Pope Francis (C) arrives to celebrate a Holy Mass in the Basilica of Our Lady of Guadalupe in Mexico on February 13, 2016. Pope Francis is in Mexico for a trip encompassing two of the defining themes of his papacy: bridge-building diplomacy and his concern for migrants seeking a better life. AFP PHOTO / GABRIEL BOUYS / AFP / GABRIEL BOUYS (Photo credit should read GABRIEL BOUYS/AFP/Getty Images)

Pope Francis (C) at an open-air mass at the Guadalupe Basilica in Mexico City on February 13, 2016. The pope urged Mexican bishops Saturday to take on drug trafficking with 'prophetic courage,' warning that it represents a moral challenge to society and the church. AFP PHOTO / Alfredo ESTRELLA / AFP / ALFREDO ESTRELLA (Photo credit should read ALFREDO ESTRELLA/AFP/Getty Images)

Pope Francis officiates an open-air mass at the Guadalupe Basilica in Mexico City on February 13, 2016. The pope urged Mexican bishops Saturday to take on drug trafficking with 'prophetic courage,' warning that it represents a moral challenge to society and the church. AFP PHOTO / Alfredo ESTRELLA / AFP / ALFREDO ESTRELLA (Photo credit should read ALFREDO ESTRELLA/AFP/Getty Images)

MEXICO CITY, MEXICO - FEBRUARY 13: Pope Francis speaks during a mass for the people at Basilica de Guadalupe on February 13, 2016 in Mexico City, Mexico. Pope Francis is on a five-day visit in Mexico from February 12 to 17 where he is expected to visit five states. (Photo by Hector Vivas/LatinContent/Getty Images)

Pope Francis (R) officiates an open-air mass at the Guadalupe Basilica in Mexico City on February 13, 2016. The pope urged Mexican bishops Saturday to take on drug trafficking with 'prophetic courage,' warning that it represents a moral challenge to society and the church. AFP PHOTO / Alfredo ESTRELLA / AFP / ALFREDO ESTRELLA (Photo credit should read ALFREDO ESTRELLA/AFP/Getty Images)

TOPSHOT - Pope Francis wears a traditional Mexican sombrero hat received as a gift by a Mexican journalist on February 12, 2016, aboard the plane to Havana.
Pope Francis headed to Cuba on Friday looking to heal a 1,000-year-old rift in Christianity before embarking on a tour of Mexico dominated by modern day problems of drug-related violence and migration. / AFP / POOL / ALESSANDRO DI MEO (Photo credit should read ALESSANDRO DI MEO/AFP/Getty Images)

Pope Francis waves from the Popemobile upon his arrival in Mexico City on February 12, 2016. Catholic faithful flocked to the streets of Mexico City to greet Pope Francis on Friday after the pontiff held a historic meeting with the head of the Russian Orthodox Church in Cuba. AFP PHOTO/Yuri Cortez / AFP / YURI CORTEZ (Photo credit should read YURI CORTEZ/AFP/Getty Images)

Pope Francis (C) waves next to Mexican President Enrique Pena Nieto (R) and First Lady Angelica Rivera (L) upon his arrival at Benito Juarez international airport in Mexico City on February 12, 2016. Catholic faithful flocked to the streets of Mexico City to greet Pope Francis on Friday after the pontiff held a historic meeting with the head of the Russian Orthodox Church in Cuba. AFP PHOTO/RONALDO SCHEMIDT / AFP / RONALDO SCHEMIDT (Photo credit should read RONALDO SCHEMIDT/AFP/Getty Images)

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The pope said he did not want to advise American Catholics on whether or not to vote for Trump, and it was not immediately clear what impact his remarks would have on Republicans likely to vote in the Nov. 8 election.

Republican Catholics appear to support Trump more than other Republicans do, according to a Reuters/Ipsos opinion poll.

As of 2014, 71 percent of the U.S. population identified as Christian, 21 percent of them Catholic, according to the Pew Research Center.

Asked if American Catholics should vote for someone with Trump's views, Francis said, "I am not going to get involved in that. I say only that this man is not Christian if he has said things like that. We must see if he said things in that way and in this I give the benefit of the doubt," he said.

Trump, a real estate developer and former reality TV star, said, "If and when the Vatican is attacked by ISIS, which as everyone knows is ISIS's ultimate trophy, I can promise you that the pope would have only wished and prayed that Donald Trump would have been president," Trump said in a speech in Kiawah Island, South Carolina, using an acronym for the Islamic State militant group.

"For a religious leader to question a person's faith is disgraceful," he said. Trump has said on occasion, and again on Thursday, that he is a proud of being Christian but emphasized early in the campaign that he does not talk about his faith as much as others might. He has said his book "The Art of The Deal" is his second most favorite book after the Bible.

"I am a very nice person. And I'm a very good Christian. Because the pope said something to the effect that maybe Donald Trump isn't Christian, ok? And he's questioning my faith. I was very surprised to see it, but I am a Christian. I'm proud of it."

Trump has said he would deport millions of illegal immigrants if he wins the White House. Last week, responding to the pope's plan to visit the U.S.-Mexican border, he said that Pope Francis did not understand the issues involved.

"The pope is a very political person. I think he doesn't understand the problems our country has. I don't think he understands the danger of the open border that we have with Mexico," Trump told the Fox Business Network.

Click through some of Donald Trump's most controversial Tweets:

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Craziest Donald Trump Tweets

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Pope says Trump 'not Christian,' Trump calls comment 'disgraceful'

I told you @TIME Magazine would never pick me as person of the year despite being the big favorite They picked person who is ruining Germany

I would like to wish everyone, including all haters and losers (of which, sadly, there are many) a truly happy and enjoyable Memorial Day!

Just made the point at #NCGOPcon that "we have to protect our border & I think everyone here knows, nobody can build a wall like Trump!"

My twitter has become so powerful that I can actually make my enemies tell the truth.

China's Communist Party has now publicly praised Obama's reelection. They have never had it so good. Will own America soon.

While Jon Stewart is a joke, not very bright and totally overrated, some losers and haters will miss him & his dumb clown humor. Too bad!

Why is Obama playing basketball today? That is why our country is in trouble!

An 'extremely credible source' has called my office and told me that @BarackObama's birth certificate is a fraud.

Remember, I said Derek don't sell your Trump World Tower apartment...its been lucky for you. The day after he sold it, he broke his foot.

I never fall for scams. I am the only person who immediately walked out of my ‘Ali G’ interview

.@cher--I don’t wear a “rug”—it’s mine. And I promise not to talk about your massive plastic surgeries that didn’t work.

The concept of global warming was created by and for the Chinese in order to make U.S. manufacturing non-competitive.

.@katyperry is no bargain but I don't like John Mayer--he dates and tells--be careful Katy (just watch!).

.@BetteMidler talks about my hair but I'm not allowed to talk about her ugly face or body --- so I won't. Is this a double standard?

Truly weird Senator Rand Paul of Kentucky reminds me of a spoiled brat without a properly functioning brain. He was terrible at DEBATE!

Hey @realDonaldTrump - In case you're looking for that tweet you deleted .. I got it right here. http://t.co/a6F98q5xGU

This very expensive GLOBAL WARMING bullshit has got to stop. Our planet is freezing, record low temps,and our GW scientists are stuck in ice

Everyone knows I am right that Robert Pattinson should dump Kristen Stewart. In a couple of years, he will thank me. Be smart, Robert.

26,000 unreported sexual assults in the military-only 238 convictions. What did these geniuses expect when they put men & women together?

Sorry losers and haters, but my I.Q. is one of the highest -and you all know it! Please don't feel so stupid or insecure,it's not your fault

How amazing, the State Health Director who verified copies of Obama’s “birth certificate” died in plane crash today. All others lived

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Asked about being called a "political person", Francis said on Thursday, "Thank God he said I was a politician because Aristotle defined the human person as 'animal politicus.' So at least I am a human person."

SOCIAL MEDIA ERUPTS

The pope's remarks and Trump's response lit up social media with many Twitter users speculating on how Trump would move on from the pope's withering comment. Author Dan Dicker @Dan_Dicker tweeted, "Let's see @realDonaldTrump insult his way out of this."

Trump accused the Mexican government of making disparaging remarks about him to the pope "because they want to continue to rip off the United States, both on trade and at the border,... and they understand I am totally wise to them."

He said the pope only heard one side of the story and did not see what Trump called the crime, drug trafficking and negative economic impact Mexico's policies had on the United States.

Evangelical Christian leader Jerry Falwell Jr, who has endorsed Trump and spent time with the candidate on his private plane, described him as generous to his employees and family, adding, "I'm convinced he's a Christian. I believe he has faith in Jesus Christ."

Reuters/Ipsos polling since the start of the year shows 43 percent of likely Republican Catholic voters support Trump, compared to 38 percent of Republican voters generally.